Spectrum allocations are carried out according to prevailing regulatory policies in different regions throughout the world. Except for small blocks of spectrum that have been set aside for unlicensed operation, spectrum is typically treated as property with associated ownership rights. Since spatial and temporal usage of spectrum varies according to the traffic demands from different services, such static allocations create an illusion of spectrum scarcity, i.e., the unavailability of spectral resources at a particular location and time. Recently, there has been considerable interest in designing Media Access Control (MAC) and physical layer techniques that allow open, opportunistic access to spectrum. This capability is desirable not only to address the long-term projected capacity demands of commercial, military and public safety applications, but also to enable unlicensed commercial or tactical military communications without any frequency planning or coordination.
Physical layer techniques, protocols and algorithms employed in evolving cellular and wireless Local Area Network technologies (e.g., IEEE 802.11) are typically designed assuming a static, contiguous spectrum allocation constraint and are often limited to narrowband operation (tens of kHz to a few MHz). In the future, advances in dynamic spectrum access techniques such as spectrum sensing and characterization, frequency agility, and dynamic radio bearer management hold the promise of huge improvements in spectrum utilization relative to current static allocations by providing the ability to exploit temporal and spatial variability in spectrum availability.
One of the outstanding challenges in ad hoc networks based on opportunistic, open spectrum access is on the design of techniques that help achieve initial neighbor discovery and association of new nodes with a neighborhood. In particular, when spectrum is not exclusively allocated to users and/or networks, users must discover the presence of neighboring users by searching across a wide range of frequencies with minimum transmit/receive power requirements and with minimum delay. This needs to be carried out in advance of opportunistic establishment of radio bearers for control and/or data transfer. Another challenge in a dynamic spectrum access framework is to ensure that the neighbor discovery process does not result in excessive interference related to co-existence with non-cooperative nodes (e.g. belonging to legacy networks). Careful consideration must be given to such co-existence scenarios when designing protocols and algorithms that enable open spectrum access.
Neighbor discovery typically involves transmission of beacons (probe messages) subject to certain criteria and also scanning for beacons from candidate neighbor nodes. These probe messages may indicate several parameters of interest including the address (or identifier) of the node, location, spectral quality measurements etc. which may be used for resource allocation, routing or forwarding decisions and energy conservation. Neighbor discovery is said to occur upon successful detection and decoding of a probe message from a cooperative node whose presence was previously unknown.
While it may be possible to improve neighbor discovery performance by limiting beacon transmissions to a fixed, pre-determined region of the spectrum, such an approach is not scalable, e.g. to support open spectrum access networks with large numbers of cooperative nodes or large numbers of co-existing networks. Furthermore, fixed regions of the spectrum are also highly susceptible to jamming or interference from non-cooperative nodes, thus making it difficult for cooperative nodes to discover each other. If cooperative nodes are not able to discover each other, then opportunistic use of spectrum for data transfer between these nodes is not possible.
The state of the art on neighbor discovery focuses on static and/or small allocations of spectrum. For example, Wireless Local Area networks based on 802.11x standards use different beacon frame transmission and reception techniques depending on the mode of operation. In an infrastructure mode, Access Points carry out periodic beacon frame transmissions on a frequency channel and all other nodes scan across different channels to detect the presence of Access Points. In an ad hoc mode, each node that is attempting discovery scans for beacons over a certain time period and transmits a beacon on a particular channel after a random delay if none are detected. The results of a performance analysis of neighbor discovery for ad hoc networks with random beacon transmission and random reception is described by L. Gallulccio, G. Marbit and S. Palazzo, in “Analytical Evaluation of a Tradeoff Between Energy Efficiency and Responsiveness of Neighbor Discovery in Self-Organizing Ad Hoc Networks,” IEEE Journal on Selected Areas in Communications, Vol. 22, No. 7, September 2004. That work was based on pre-fixed frequency carrier sets and does not apply to a dynamic spectrum access framework where frequencies deemed acceptable for beacon transmission and/or reception may vary from node to node according to the perceived spectral quality. Furthermore, no consideration was given to policy constraints associated with opportunistic open access wireless networks.